1. Field of the Invention:
This invention relates generally to a viscous fluid coupling device adapted for use in motor vehicles, and more particularly to a viscous fluid coupling device which controls the number of rotations of the radiator cooling fan in response to the temperature of the engine.
2. Description of the Prior Art:
Conventionally, thermally controlled viscous fluid coupling devices generally include an operating chamber for accommodating a rotor and a reservoir chamber for storing the operating oil. The revolutions per minute of the radiator cooling fan is controlled by means of supplying the operating oil from the operating chamber to the reservoir chamber and reducing the quantity of the operating oil in the operating chamber when the temperature of the engine is low, and by means of returning the operating oil supplied to the reservoir chamber and increasing the quantity of the operating oil in the operating chamber when the temperature of the engine is high. A valve plate rotated by a bimetal element is axially attached to a partition plate dividing the interior into the operating chamber and the reservoir chamber. The quantity of the operating oil returning from the reservoir chamber to the operating chamber is controlled by the valve plate, which acts to open and close an oil returning hole formed in the partition plate.
In a conventional viscous fluid coupling device of the thermally controlled type, however, there is only one mechanism for an oil returning hole and one mechanism for a pump hole. Accordingly, since the rotation of the radiator cooling fan is controlled by the temperature of the engine in two steps, including an ON-state and an OFF-state, the rotation of the radiator cooling fan cannot be precisely and accurately controlled in response to the temperature of the engine.